1. Field of the Invention
This invention relates to an improved method of operating a regeneration zone of catalytic cracking units.
2. Description of the Prior Art
Environmental limitations imposed by state and federal regulatory agencies are becoming increasingly important considerations in the operation of catalytic cracking units (e.g., fluid catalytic cracking --FCC units). In many areas of the country, and even in some foreign countries, economic penalties, (e.g., reduced throughput, more expensive raw materials) are being paid for the excessively high levels of pollutants produced in the catalytic cracking operations.
A typical FCC unit comprises a reactor zone or vessel filled with a catalyst, and a regenerator vessel wherein spent catalyst is regenerated. Feed is introduced into the reactor vessel, and is converted therein over the catalyst. Simultaneously, coke forms on the catalyst and deactivates the same. The deactivated (spent) catalyst is removed from the reactor zone and is conducted to the regenerator zone, wherein coke is burned off the catalyst with an oxygen-containing gas (e.g., air), thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. Some of the catalyst is fractionated into fines and lost during the process because of constant abrasion and friction thereof against the various parts of the apparatus. Most of the gaseous pollutants, formed in a catalytic cracking operation, are produced in the regenerator zone or vessel.
The efficiency of the regenerating operation is dependent on several operating parameters, the most important of which are regeneration temperature and oxygen availability. In recent years most operators have concentrated on raising regenerator temperature to increase the efficiency of the regenerator zone through a complete or almost complete combustion of carbon monoxide in the regenerator vessel. This is most commonly accomplished by operating at air rates exceeding those required to burn the coke off the catalyst, and by introducing a carbon-monoxide (CO) combustion promoter, usually comprising at least one of the following metals: platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os), and rhenium (Re). Some new regenerator designs have incorporated better mixing methods for mixing coked catalyst with a CO combustion promoter and oxygen (e.g., fast fluidized bed regenerator of Gross et al, U.S. Pat. No. 4,118,338, the entire contents of which are incorporated herein by reference). However, while these new methods of operation of the regenerating vessel decrease the amount of carbon monoxide exiting with the flue gas and improve the overall efficiency of the regeneration process, they may contribute to an increased level of production of other pollutants, e.g., sulfur oxides, particularly sulfur trioxide (SO.sub.3), and nitrogen oxides (see for example Luckenbach, U.S. Pat. No. 4,235,704).
Simultaneously with the improved methods of operation of the regeneration zone, which alone may contribute to the increased production of sulfur oxides in the flue gases of the regenerator, sulfur feed levels in petroleum crudes available for cracking have been steadily increasing over the past few years. In the past, due to overall low levels of sulfur in FCC feeds, SO.sub.3 levels in flue gases were low and generally only total SO.sub.x levels were monitored without an SO.sub.2 /SO.sub.3 breakdown, or without regard to the SO.sub.3 levels. With the combination of the high sulfur feed levels, the high temperatures in the regeneration zone, and excessive air rates used in the regenerator, the SO.sub.3 concentration in the flue gas can be high enough to cause condensation in the flue gas which can result in a visible plume. The presence of a visible plume may violate local opacity requirements. In addition, the absence of a plume indicates, in a vast majority of cases, that the SO.sub.3 emissions have not reached the maximum allowable limit.
The excessively high levels of SO.sub.x and SO.sub.3 are particularly experienced when it is necessary to reduce throughput, as required, for example, by seasonal shifts in demand for FCC products, or shifts due to upstream or downstream processing problems. The SO.sub.3 levels are particularly high under those circumstances because the modern regenerator designs (e.g., those of U.S. Pat. No. 4,118,338) require gas velocities in the combustor section sufficiently high to provide entrainment rates greater than the catalyst circulation rate. However, as mentioned above, the operation of the regenerator at such high oxygen-containing gas velocities, at reduced throughputs, results in excessive levels of sulfur oxides (SO.sub.x), and especially sulfur trioxide (SO.sub.3), in the flue gas. Such high levels of SO.sub.x may violate current environmental regulations.